Get rid of the one remaining extra call to LayerPainter::beginTransparencyLayers.

This allows us to get rid of a related state bit on RenderLayer as well.

Review URL: https://codereview.chromium.org/727633002

git-svn-id: svn://svn.chromium.org/blink/trunk@185402 bbb929c8-8fbe-4397-9dbb-9b2b20218538